The present invention relates to humidity control systems. More particularly, the present invention relates to a dehumidifier comprising a housing containing one or more hygroscopic monoliths that is capable of providing and passively sustaining the capacity to dehumidify air. The present invention may be employed either independently or as part of a larger or more extensive climate control system.
Typical atmospheric desiccant dehumidifiers employ solid or liquid desiccant to dry working gases, such as removing water vapor from air. Typical solid desiccant dehumidifiers employ desiccant impregnated wheels, columns packed with randomly dispersed desiccants, horizontal desiccant bed trays or multiple vertical beds. Before a desiccant dehumidifier can dry a process air stream, water must be removed from the desiccant by drying or activation induced by, for example, a drying air stream. The drying air stream may employ heat to raise the vapor pressure on the surface of the desiccant causing desorption so that the evolved water vapor can be carried away. Alternatively, the drying stream can comprise dry air having a low water vapor pressure relative to the surface of the desiccant such that desorption is promoted. The activated desiccant must then either be used to modify a process air stream or be isolated by some means, typically with valves or other control mechanisms, to prevent significant reduction in its drying capacity due to exposure to potentially damp ambient conditions. Desiccant dehumidifiers that incorporate desiccant wheels, horizontal bed trays or multiple vertical beds typically work continuously without the ability to retain their drying capacity for intermittent use. Dehumidifiers using columns packed with randomly dispersed desiccant can isolate the activated desiccant with valves affixed at either end of the column to allow intermittent use, but such designs are limited by the pressure drop induced by turbulence created while moving air through the packed column. Further limitations are imposed by the expense of valves and other control components necessary for large air flows as well as the support and retention components required in the packed desiccant columns.
The present invention overcomes limitations inherent to existing systems by incorporating, as a desiccant, one or more hygroscopic monoliths shaped and arranged in such a manner that air is permitted to flow through the desiccant housing without the relatively high pressure resistance or turbulence induced by existing packed columns. The unique shape and arrangement of the hygroscopic monoliths of the present invention also induce a xe2x80x9cchimneyxe2x80x9d effect which allows the dehumidification capacity of the desiccant to be passively preserved, not by the use of valves or control mechanisms, but by allowing the vapor pressure at the surface of the monoliths to approach that of the surrounding air, at which time the drying or adsorption process slows, or even stops, as a state of equilibrium is achieved. In this way, the dehumidification capacity permitted by the present invention can be controlled simply by regulating the airflow through the system without the need to physically isolate the desiccant during non-use. The present system may alternatively operate as a humidifier with the minor modifications described herein.
Accordingly, the present invention provides a solid desiccant humidity control system that permits flow across a desiccant of, alternatively, a drying air stream or a process air stream with relatively little resistance and pressure drop due to the shape and arrangement of the desiccant. One or more hygroscopic monoliths are incorporated, as a desiccant, into the present invention in a manner that induces a xe2x80x9cchimneyxe2x80x9d effect within and around the monoliths, thereby permitting the dehumidification capacity of the desiccant to be passively preserved without the use of valves or control mechanisms by allowing the vapor pressure at the surface of the monoliths to approach that of the surrounding air, at which time the drying or adsorption process slows, or even stops, as a state of equilibrium is achieved. Because of this xe2x80x9cchimneyxe2x80x9d effect and the state of equilibrium it encourages, the dehumidification capacity of the desiccant employed in the present invention can be preserved simply by regulating the airflow through the system without the need to physically isolate the desiccant during periods of non-use.
The drying air stream may be air at an elevated temperature relative to the process stream or an air stream with a water vapor pressure lower than that of the process stream. Thus, the present invention provides a dehumidifier that can be utilized in a wide variety of settings, including incorporation into a wide variety of climate control systems, due to the ease of flow through the unit and its ability to operate intermittently in response to dehumidification demand or the availability of a drying air stream that reactivates the desiccant. By using a charging stream in lieu of a drying stream, the present invention may alternatively serve as a humidifier. The present invention comprises a housing having a cavity containing a hygroscopic material, such as one or more hygroscopic monoliths. In an exemplary embodiment of the invention, the housing defines a cavity with inlet and outlet ports located in the base of the housing. Except for the inlet and outlet ports, the housing is sealed at its top, its bottom and along all sides, although optional, sealed access may be provided for sensors, gauges and the like if desired. Disposed or positioned within the housing is an array of hygroscopic monoliths, such as the kind shaped to facilitate circulation and drying by means of a xe2x80x9cchimneyxe2x80x9d effect as disclosed in U.S. patent application Ser. No. 09/157,208, filed Sep. 18, 1998, entitled xe2x80x9cHygroscopic Monolith Having a Channel Therethrough for Modifying a Gas Therein by Adsorption or Desorption, and Processes Therefore,xe2x80x9d the disclosure of which is incorporated by reference as if set forth herein in its entirety. Such application discloses a variety of shapes useful for achieving the beneficial effects imparted by the present invention. In addition, hygroscopic monoliths shaped as cubes having a single cylindrical channel, as disclosed herein, may be utilized in the present invention.
The hygroscopic monoliths may be composed of, for example, zeolite, or any similar hygroscopic material. The hygroscopic monoliths are situated within the housing such that a drying air stream can, with relatively low resistance or pressure drop, flow across the external surfaces of the monoliths as well as through channels disposed among the monoliths. Elastomeric Hi gaskets create passages within the housing to direct the drying stream, and alternatively the process stream, to and from the monoliths. A drying air stream may be obtained from a wide variety of sources, including solar thermal, geothermal, waste or vented heat, combustion, electrical resistance, heat pipes, heat pumps or other dehumidifiers.
After undergoing activation, the hygroscopic monoliths gradually dry the air contained within the housing and the drier air, being more buoyant, tends to rise within the housing. The activated monoliths therefore promote air circulation within the housing, generally filling the cavity from the top down with the driest portion of the contained air. The monoliths will continue to promote circulation of the air contained within the housing until the vapor pressure at the surface of the monoliths approaches that of the surrounding air, at which time the drying or adsorption process slows, or even stops, as a state of equilibrium is achieved. In this way, the present invention possesses the ability to retain its drying capability by isolating the monoliths from the process air stream and ambient air external to the housing through passive means during cooling or inactivity between periods of intermittent use.
A dehumidifier according to the present invention may have various configurations of enclosure surfaces, port locations, passageway orientations and hygroscopic monolith shapes to permit selective utilization of the buoyant forces within the enclosure, the available drying stream and any other performance-modifying aspects attributable to a particular setting. Airflow through the ports can be induced by suction, positive air pressure or a combination of both suction and pressure. Airflow may also be induced or assisted by orienting the housing such that buoyant forces expel drier air out of the housing while drawing relatively heavier, water-vapor-laden ambient air into the housing.
The present invention may alternatively be utilized as a humidifier if ports are located at the top of the housing and, in lieu of a drying stream, a charging air stream having a lower temperature or higher water vapor pressure than the process stream flows across the desiccant, thereby promoting adsorption of water vapor upon contact of the charging stream with the monoliths. In the absence of a charging or process air stream, the air within the housing interacts with the charged monoliths and, in so doing, becomes progressively more laden with water vapor. This moisture-laden air, being heavier relative to drier air, generally tends to settle around the monoliths until the vapor pressure at the surface of the monoliths approaches that of the surrounding air and desorption slows, or even stops, as a state of equilibrium is achieved. In this way, a system comprising the present invention possesses the ability to retain its humidification capability by isolating the monoliths from the process air stream through passive means during cooling or inactivity between periods of intermittent use.
The dehumidifying capability provided by the present invention may be employed either independently or as a component of a larger unit. Similarly, the humidifying capability provided by the present invention may also be employed either independently or as a component of a larger unit.